Aboveground and belowground competition affect seedling performance and allometry in a tropical monsoon forest

Distinguishing the relative effects of above- and belowground competition can improve our understanding of the forces shaping community assembly in different ecosystems. In this study, we investigated the impacts of above-and belowground competition on seedling growth and allometry in a tropical monsoon forest (TMF) on Hainan Island, China. Four common dominant canopy tree species from three forest types (Schima superba in secondary forest, Peltophorum tonkinense and Vatica mangachapoi in lowland rainforest, and Terminalia nigrovenulosa in TMF) were planted in different treatments (understory vegetation removal and trenching to decrease above-and belowground competition, respectively). Three species had greater relative growth rates in height in the vegetation removal, but not in the trenching treatment. Vegetation removal reduced mortality rates for all species and increased biomass in three of the four. Trenching alone did not affect biomass for any species. Vegetation removal affected seedling allometry, resulting in higher leaf mass fraction in the lowland rainforest species P. tonkinense and V. mangachapoi. The secondary forest species, S. superba, did not successfully establish in plots with intact understory because of aboveground competition. The highly droughtadapted traits of T. nigrovenulosa allowed its seedlings to establish successfully in TMF, where it is a canopy dominant. Our results demonstrate that aboveground competition is more important than belowground competition in structuring seedling communities in TMF, because light availability largely limits seedling establishment in this ecosystem.